Ion illumination angle control for a mass spectrometer

ABSTRACT

A more efficient operation of a spark source mass spectrometer is achieved by adjusting the position of the spark gap with respect to the ion optical system as well as the spark gap width. The adjustment of the position of the spark gap provides a more nearly constant ion beam axis ion illumination angle so that a more constant instrument sensitivity is maintained.

United States Patent [191 Conzemius Oct. 7, 1975 1 ION ILLUMINATION ANGLE CONTROL 3,5l8,424 6/1970 Wanless et al 250/281 x FOR A MASS SPECTROMETER 3,686,683 8/1972 Powers et a]. 250/426 X Inventor: Robert J. Conzemius, Ames, Iowa Assignee: The United States Energy Research & Development Administration, Washington, DC.

Filed: Apr. 29, 1974 App]. No.: 464,889

U.S. Cl 250/281; 250/423 Int. Cl. H01J 39/34 Field of Search 250/281, 282, 283, 284,

References Cited UNITED STATES PATENTS 10/1952 Perkins et al. 250/283 Primary ExaminerDavis L. Willis Attorney, Agent, or FirmDean E. Carlson; Arthur A. Churm; Walter L. Rees [57] ABSTRACT A more efficient operation of a spark source mass spectrometer is achieved by adjusting the position of the spark gap with respect to the ion optical system as well as the spark gap width. The adjustment of the position of the spark gap provides a more nearly constant ion beam axis ion illumination angle so that a more constant instrument sensitivity is maintained.

6 Claims, 6 DrawingFigui-es U.S. Patent 0a. 7,1975' Sheet 1 of4 3,911,272

E S EV NEE.

EDEQIEOM ZO JOEFZOU M4024 U.S. Patent Oct. 7,1975 Sheet 2 of 4 3,911,272

X AXIS US. Patent Oct. 7,1975 Sheet 4 of4 3,911,272

RELATIVE POSITION 0F LOWER TO UPPER SPARK SAMPLES ION ILLUMINATION ANGLE CONTROL FOR A MASS SPECTROIVIETER CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION The spark source mass spectrometer is an extremely useful analytical tool because of its ability to provide semiquantitative assays for essentially all elements with very high detection sensitivity. Significant improvements have been made in the instrument and the tech nique of using it. One of the more significant improvements was the development of a method of maintaining automatically a spark via continuous adjustment of the spark gap width. However, control of the spark gap width alone is not sufficient to maintain the optimum spark electrode geometry.

It has been qualitatively established that the ion beam from the spark source enters the ion optical system highly dependent on the actual location of the spark relative to the bulk shape of the sparking electrodes, the gap width and the coordinates of the ion optical system. The Z-axis spatial distribution of the ions from the ion optical axis as the ions travel through the ion optical system is the Z-axis ion illumination angle (ion illumination angle). Ordinarily, spark electrode shape and gap geometry can be chosen initially for full illumination of the object slit. Although the spark gap width can then be maintained automatically, the spark location may soon become less than ideal and may even become hidden from the ion optical system by the electrodes themselves due to the erosion of one or both of the electrodes. This results in a degradation of the efficiency and sensitivity of the system or the failure of the spectrometer to operate.

It is therefore an object of this invention to provide an improved spark source mass spectrometer.

Another object of this invention is to provide a structure for a spark source mass spectrometer which automatically adjusts the ion illumination angle of the spectrometer.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated in the drawings, of which FIG. I is a schematic view of the structure of a spark source mass spectrometer incorporating the features of this invention; I

FIG. 2 shown the electrode and limiting plate structure;

FIG. 3 is a block diagram of the electric circuits which provide the automatic control features of the invention;

FIG. 4 is a graph showing the result of changing the ion illumination angle;

FIG. 5 is a drawing showing theoperation of the automatic spark adjustment; and

FIG. 6 is a graph showing the results of using the ion illumination control system.

SUMMARY OF THEINVENTION The spark source mass spectrometer includes spark source electrodes across which a spark is developed. One or both of the electrodes contain the material being examined so that ions of this material are formed by the spark. These ions are directed along an ion beam axis by an ion beam forming and accelerating structure which includes a lens system for defining the ion beam and directing the same along an ion beam axis from the spark source to the ion detector. The spectrometer includes structure for adjusting the spark gap width so as to maintain an optimum width. Limiting plates are positioned on opposite sides of the ion beam so as to intercept ions outside of the main beam portion of the ion beam. The ions striking the limiting plates are used to develop ion beam signals which are used to control the ion beam illumination angle. This control is achieved by moving one electrode relative to the other electrode parallel to the ion beam axis in response to the ion signals so as to maintain a predetermined ratio value between the ion beam signals from each of the limiting plates.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a schematic drawing of a spark source mass spectrometer. The spectrometer has three axes, as shown at 10, the X-axis along which the ion beam is directed, a Y-axis and a Z- axis. All three axes are mutually orthogonal. Ions are developed at the spark gap whichis between electrodes 11 and 13. As shown in the drawing by the dotted line positions, electrode 11 can be moved relative to electrode 13 between electrode positions 11a and 11b, as indicated by arrow 31. Electrode 11 can also be moved relative to electrode 13 in the direction indicated by arrows 32 to adjust the gap width.

The ion beam is accelerated along the X-axis (the ion beam axis) by the high-voltage accelerating plate 14 and the grounded slit 16 and passes through the cylindrical electrostatic analyzer 17. The ion beam passes the upper Z limit plate 19 and the lower Z limit plate 20 and the total beam monitor 23. Electrodes 22 are for the purpose of secondary electron suppression. The ion beam then passes through the magnetic analyzer 25 to an ion detector 29. Electrode 26 is a grounded slit and electrode 28 is for the purpose of secondary electron suppression. With the exception of the relative movement of the sparking electrodes 11 and 13, the spark source mass spectrometer of FIG. 1 is basically a standard spark source mass spectrometer such as the Nuclide Corporation Model Graf II-2 spark source mass spectrometer.

Referring to FIG. 2, there is shown an enlarged view of the spark source electrode configuration and the Z- axis limiting plates 19 and 20 relative to the X, Y and Z ion optical axes. Portions of the structure which are identical to those in FIG. 1 have the same reference numerals. The electrodes 11 and 13 are formed of the material which is being analyzed or a small portion of the electrode can be so formed. In some cases, the material being analyzed is held by the electrode and forms part of the circuit which develops the spark. The energy of the spark source causes the material being analyzed to ionize and these ions are formed into an ion beam and are directed along the X-axis (the ion beam axis). Most of the ions so accelerated are directed along a main beam portion of the ion beam and are eventually detected by the ion detector 29 of FIG. 1. However, a small portion of the ions in the ion beam are outside of the main beam portion and some of these ions are intercepted by the Z limit plates 19 and 20 and are used to develop output signals which are a function of the numher of ions striking plates 19 and 20. These output signals are used to change the relative position of electrodes 11 and 13.

Referring to FIG. 3, there is shown a block diagram of the electronic circuitry used in controlling the spark source mass spectrometer. The current used to form the spark is in the form of a pulse which has a variable width which may be (for example) from 3,2 to 320 microseconds at an alternating high voltage of to 80 kv and l megahertz. The repetition rate of the pulse is also variable, for example, from 1 hertz to 3.2 kilohertz. This current is supplied by high-voltage transformer 36. An electromagnetically coupled circuit, transformer 37, senses the magnitude of the spark gap voltage. The signal from transformer 37 is rectified in rectifier 39 to provide a D-C voltage which is proportional to the spark gap voltage and thus the spark gap width. The D-C signal is filtered in the variable time constant circuit 40 and amplified in amplifier 42. A second input to amplifier 42 is a reference voltage 44. The output from amplifier 42 is in the form of an error signal (the difference between the reference signal and the signal from filter 40) which drives a motor 43 to vary the spark gap width, thereby maintaining the spark gap at a desired distance.

As previously described in connection with FIG. 1, the ions which are outside the main beam in the direction of the Z-axis strike the upper and lower Z-axis limit plates 19 and 20. These plates again are shown in FIG. 3. The charges built up on the upper and lower Z limit plates as a result of the ions striking these plates are detected by electrometers 47 and 48 which develop output signals which are a function of the number of ions striking the plates. These signals are coupled to summing circuit 50 where they are summed to develop a beam reference signal in circuit 51. The output from electrometer 47 is coupled to ratio select circuit 53, while the output of electrometer 48 is coupled directly to variable time constant circuit 54. The ratio select circuit 53 is essentially an attenuation circuit or a potentiometer circuit which selects a portion of the signal from electrometer 47. The attenuated signal from the ratio selector 53 is also coupled to variable time constant circuit 54 which acts to filter both of the signals applied thereto. The filtered signals are coupled to operational amplifier circuit 57 which develops an output error signal according to the difference between the signals. The error signal is thus a function of the ratio between the ions striking the upper Z-axis limit plate and the lower Z-axis limit plate. The output of amplifier 56 is coupled to a motor 58 through a beam off disable circuit 57. The signal applied to motor 58 acts to position the electrodes along the X-axis as required to maintain a constant Z-axis ion illumination angle. The output of summing circuit 50 is also coupled to beam off disable circuit 57. The beam off disable circuit 57 acts as a switch to disable the motor drive circuit 58 in the event that no beam is present.

Referring to FIG. 4, curve 60 illustrates the change in the ratio between the ion signal on the upper Z-axis limiting plate and the ion signal on the lower Z-axis limiting plate as the relative X-axis position of the electrodes is changed. The relative position of the electrodes is also shown in schematic form at the bottom of the graph. Curve 61 shows how the relative values of ion current measured at the image of the instrument change as the ion illumination angle setting or the relative position of the electrodes is changed. Curve 61 shows that the transmission of ions to the image has a rather broad maximum but decreases sharply at the extremes of the ion illumination angle setting. Similarly shaped curves are obtained for single as well as doubly charged ions. As previously described, these signals from the upper and lower Z-axis limiting plates are utilized for adjustment of one electrode relative to the other such that optimum illumination at the image of the optical system is maintained.

Referring to FIG. 5, there is shown the electromechanical system which is used to position the electrodes. Electrode 13 is positioned inside the spark source mass spectrometer (not shown) in a vacuum. Electrode 11 is also inside the spark source mass spectrometer and positioned adjacent and apart from electrode l3. Electrode 11 is connected to a rod 65 which extends outside the spark source mass spectrometer through a vacuum bellows seal 66. Rod 65 is coupled to a positioning block 69 through a ball joint 68. Rod 65 is pivoted at ball joint 71 and a Y-axis positioning adjustment can be made through the screw structure 72. Drive motor 58 is coupled to a rack and pinion structure 74 to move the rack and pinion structure as shown by arrows 75. This moves a positioning block 69 in the direction of arrows 75. Motor 43 is coupled to the positioning block 69 through rack and pinion structure 77 to move the positioning block in the direction shown by arrows 78.

As can be seen in FIG. 5, the drive motor 43 moving the positioning block in the Z-axis direction, that is the direction shown by arrows 78, will cause electrode 11 to move in the Z-axis direction, thus adjusting the gap width at the electrodes 11 and 13. Similarly, the drive motor 58 moving the positioning block 69 in the direction shown by arrows 75 will move electrode 11 along the X-axis to change the Z-axis ion illumination angle, as previously described.

Referring to FIG. 6, there is shown a recorder plot of the output signal from the spark source mass spectrometer using the Z-axis ion illumination angle control structure of this invention. After the start with the Z- axis ion illumination angle properly adjusted, the signal was relatively good for several minutes and then, as illustrated in the figure, the signal became erratic due to electrode erosion. At about the time 3 minutes, the signal became practically unusable. At this point, the Z- axis ion illumination angle control was turned on and the position of the electrodes was changed automatically by the Z-axis ion illumination control device. After about a minute of operation the signal reached an equilibrium and from this point on was maintained at this condition automatically by the Z-axis ion illumination control device of this invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A device for controlling automatically the ion illumination angle in a spark source mass spectrometer which includes a spark source, a power supply coupled to said spark source for supplying a current thereto for developing ions, an ion detector and an ion beam forming and accelerating structure including a lens system for defining the ion beam and directing the same along an ion beam axis from the spark source to the ion detector, said controlling device comprising a first limiting plate positioned on one side of the ion beam and a second limiting plate positioned on the opposite side of the ion beam, said first and second limiting plates acting to intercept ions outside a main beam portion of the ion beam, said spark source including a first movable electrode and second electrode spaced apart from said first electrode, said first and second electrodes being coupled to the power supply and being responsive to the current supplied thereby to develop ions for the ion beam, a control circuit coupled to said first and second limiting plates and responsive to said ions intercepted by said first and second limiting plates to develop a first control signal which is a function of the ratio of said ions intercepted by said first and second limiting plates, electrode control means coupled to said control circuit and said first electrode and responsive to said first control signal to move said first electrode in a direction parallel to the ion beam axis to maintain said ratio of said ions intercepted by said first and second limiting plates at a predetermined value and thereby maintain the ion illumination angle at a desired value.

2. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 1 wherein said control circuit includes first and second detection circuits coupled to said first and second limiting plates, respectively, to develop first and second ion signals in response to the number of said ions intercepted by said limiting plates, an attenuating circuit coupled to one of said first and second detection circuits for attenuating one of said first and second ion signals relative to the other, an error signal amplifier circuit coupled to said electrode control means and to the other of said first and second detection circuits and said attentuating circuit for developing said first control signal which is a function of the difference between said one attenuated ion signal and said other ion signal.

3. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 2 wherein, said control circuit includes a summing circuit coupled to said first and second detection circuits for developing a sum signal in response to said first and second ion signals, a disabling circuit coupled to said summing circuit and coupling said error signal amplifier circuit to said electrode control means, said disabling circuit being responsive to said sum signal to couple said first control signal to said electrode control means.

4. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 3 wherein, said detection circuits are electrometers.

5. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 4 wherein said electrode control means includes a first motor coupled to said disabling circuit and a first mechanical structure mechanically coupling said first motor to said first electrode, said first motor being responsive to said first control signal to drive said first mechanical structure whereby said first electrode is moved in a direction parallel to said ion beam axis.

6. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 1, further including a third detection circuit coupled to the power supply for measuring the current supplied to the spark source and developing a second control signal which is a function of the magnitude of the current, a second motor coupled to said third detection circuit and second mechanical structure mechanically coupling said second motor to said first electrode, said second motor being responsive to said second control signal to drive said mechanical structure, whereby said first electrode is moved in a direction perpendicular to said ion beam axis to maintain the current at a predetermined value. 

1. A device for controlling automatically the ion illumination angle in a spark source mass spectrometer which includes a spark source, a power supply coupled to said spark source for supplying a current thereto for developing ions, an ion detector and an ion beam forming and accelerating structure including a lens system for defining the ion beam and directing the same along an ion beam axis from the spark source to the ion detector, said controlling device comprising a first limiting plate positioned on one side of the ion beam and a second limiting plate positioned on the opposite side of the ion beam, said first and second limiting plates acting to intercept ions outside a main beam portion of the ion beam, said spark source including a first movable electrode and second electrode spaced apart from said first electrode, said first and second electrodes being coupled to the power supply and being responsive to the current supplied thereby to develop ions for the ion beam, a control circuit coupled to said first and second limiting plates and responsive to said ions intercepted by said first and second limiting plates to develop a first control signal which is a function of the ratio of said ions intercepted by said first and second limiting plates, electrode control means coupled to said control circuit and said first electrode and responsive to said first control signal to move said first electrode in a direction parallel to the ion beam axis to maintain said ratio of said ions intercepted by said first and second limiting plates at a predetermined value and thereby maintain the ion illumination angle at a desired value.
 2. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 1 wherein said control circuit includes first and second detection circuits coupled to said first and second limiting plates, respectively, to develop first and second ion signals in response to the number of said iOns intercepted by said limiting plates, an attenuating circuit coupled to one of said first and second detection circuits for attenuating one of said first and second ion signals relative to the other, an error signal amplifier circuit coupled to said electrode control means and to the other of said first and second detection circuits and said attentuating circuit for developing said first control signal which is a function of the difference between said one attenuated ion signal and said other ion signal.
 3. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 2 wherein, said control circuit includes a summing circuit coupled to said first and second detection circuits for developing a sum signal in response to said first and second ion signals, a disabling circuit coupled to said summing circuit and coupling said error signal amplifier circuit to said electrode control means, said disabling circuit being responsive to said sum signal to couple said first control signal to said electrode control means.
 4. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 3 wherein, said detection circuits are electrometers.
 5. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 4 wherein said electrode control means includes a first motor coupled to said disabling circuit and a first mechanical structure mechanically coupling said first motor to said first electrode, said first motor being responsive to said first control signal to drive said first mechanical structure whereby said first electrode is moved in a direction parallel to said ion beam axis.
 6. The device for controlling automatically the ion illumination angle in a spark source mass spectrometer of claim 1, further including a third detection circuit coupled to the power supply for measuring the current supplied to the spark source and developing a second control signal which is a function of the magnitude of the current, a second motor coupled to said third detection circuit and second mechanical structure mechanically coupling said second motor to said first electrode, said second motor being responsive to said second control signal to drive said mechanical structure, whereby said first electrode is moved in a direction perpendicular to said ion beam axis to maintain the current at a predetermined value. 